Purification of molten metal, especially molten steel, by subjecting the molten metal to a vacuum has been known for some time. In such a process, the molten metal is poured into an open receptacle, or “ladle”, and covered with a layer of fused (liquid) mineral slag, which both insulates and isolates the molten metal, and is chemically formulated to aid the purification process. The ladle is positioned within a degassing chamber connected to a vacuum pumping arrangement for evacuating the chamber. The pumping arrangement typically comprises one or more primary pumps for exhausting gas drawn from the chamber to atmosphere, and one or more secondary mechanical vacuum booster pumps connected between the primary vacuum pumps and the degassing chamber. The pumping arrangement is operated to subject the chamber to a steadily decreasing pressure (increasing vacuum), which causes gaseous and metallic impurities to leave the liquid phase and be evacuated from the atmosphere above the melt.
However, as the pressure reduces a point may be reached at which vigorous chemical reactions occur at the interface between the molten metal and the molten slag, causing a rapid generation of gas that quickly inflates the slag layer by foaming. If uncontrolled, the foaming slag can rise up and overflow from the lip of the ladle, resulting in major loss of slag and potential disruption to the purification process.